Biogeochemical Reduction Processes in a Hyper-Alkaline Leachate Affected Soil Profile
Ian T. BurkeRobert J.G. MortimerShanmugam PalaniyandiRobert A. WhittlestonCindy L. LockwoodDavid J. B. AshleyDouglas I. Stewart
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Abstract:
Hyperalkaline surface environments can occur naturally or because of contamination by hydroxide-rich wastes. The high pH produced in these areas has the potential to lead to highly specialized microbial communities and unusual biogeochemical processes. This article reports an investigation into the geochemical processes that are occurring in a buried, saturated, organic-rich soil layer at pH 12.3. The soil has been trapped beneath calcite precipitate (tufa) that is accumulating where highly alkaline leachate from a lime kiln waste tip is emerging to atmosphere. A population of anaerobic alkaliphilic bacteria dominated by a single, unidentified species within the Comamonadaceae family of β-proteobacteria has established itself near the top of the soil layer. This bacterial population appears to be capable of nitrate reduction using electron donors derived from the soil organic matter. Below the zone of nitrate reduction a significant proportion of the 0.5N HCl extractable iron (a proxy for microbial available iron) is in the Fe(II) oxidation state, indicating there is increasing anoxia with depth and suggesting that microbial iron reduction is occurring. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental files.Keywords:
Biogeochemical Cycle
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Analysis of soil pattern, especially the pattern of depletion and accumulation zones, is a powerful tool to decipher pedogenic processes at the landscape scale. To clarify the pedogenesis of a distinct pattern of podzolized soils in the cool, perhumid Black Forest (Germany) we performed a study in the forested upper part of a granitic catchment (B a ¨ rhalde). From detailed soil mapping we selected a typical catena of four pedons, which were analyzed for physical (bulk densities and particle‐size distribution), chemical (pH, organic C, pyrophosphate, oxalate, dithionite, and total Al, Mn, Fe), and mineralogical (clay minerals) properties. Standard mass balance calculations were modified to include a two‐component system with regard to parent material. Results showed a shift from two‐mica granite to granite–porphyry downslope. Soil pattern revealed podzolized soils with thick E horizons and thin spodic horizons developed in the upslope areas, whereas in downslope soils the reverse was found (thick spodic B and thin E horizons). Soil chemical and mineralogical properties were linked to soil morphology in that contents of organic C, pedogenic oxides, hydroxy‐interlayered vermiculites (HIV), and pH increased from eluvial to illuvial horizons as well as from up‐ to downslope soils. Mass balances of Fe and Al showed negative fluxes in upslope soils and positive fluxes in downslope soils during pedogenesis. We concluded from these results that a catenary eluvial–illuvial sequence (lateral podzolization) develops as a result of upslope mobilization followed by a (partially) lateral transport and subsequent immobilization in downslope zones, probably because the base‐richer parent material built up a geochemical barrier.
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A study of tepetates found in the Glacis de Buenavista, Morelos is hereby presented. The section of study (Ahuatenco) includes a sequence of seven indurate horizons (tepetate layers), underlying a polygenetic and welldeveloped Luvisol. In this work tepetates are considered as producis of erosion-sedimentation and pedogenetic processes. Therefore they are interpreted as paleosols with different degrees of development. Macro and micromorphologic evidences indicate that the tepetate profile is constituted by a mixture of fresh coarse volcanic material, redeposited fragments of soil and pedogenetic features, which include: clay cutans, Fe-Mn nodules, weathered primary minerals and phytoliths accumulated wlthin the layer matrix. These properties show that tepetates were exposed at the surface for a sufficient time to allow pedogenesis. The tepetates' parent material are volcanic ashes derived from volcanic events, and eroded Bt-horizon remains from the upper portions that were redeposited along with volcanic materials. Because of the presence of these Bt-horizon fragments the clay content in indurate materials is high (47%), but they display a contrasting distribution, showing lithological and textural discontinuities, so each layer is likely to represent a different stage of landscape evolution. The degree of alteration observed for the different layers vary, as well as the stability periods under which pedogenesis occurs.
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